Electroluminescent displays are advantageous by virtue of their low operating voltage with respect to cathode ray tubes, their superior image quality, wide viewing angle and fast response time over liquid crystal displays, and their superior gray scale capability and thinner profile than plasma display panels.
As shown in FIGS. 1 and 2, an electroluminescent display has two intersecting sets of parallel electrically conductive address lines called rows (ROW 1, ROW 2, etc.) and columns (COL 1, COL 2, etc.) that are disposed on either side of a phosphor film encapsulated between two dielectric films. A pixels is defined as the intersection point between a row and a column. Thus, FIG. 2 is a cross-sectional view trough the pixel at the intersection of ROW 4 and COL 4, in FIG. 1. Each pixel is illuminated by the application of a voltage across the intersection of row and column.
Matrix addressing entails applying a voltage below the threshold voltage to a row while simultaneously applying a modulation voltage of the opposite polarity to each column that bisects that row in two. The voltages on the row and the column are summed to give a total voltage in accordance with the illumination desired on the respective sub-pixels, thereby generating one line of the image. An alternate scheme is to apply the maximum sub-pixel voltage to the row and apply a modulation voltage of the same polarity to the columns. The magnitude of the modulation voltage is up to the difference between the maximum voltage and the threshold voltage to set the pixel voltages in accordance with the desired image. In either case, once each row is addressed, another row is addressed in a similar manner until all of the rows have been addressed. Rows which are not addressed are left at open circuit.
The sequential addressing of all rows constitutes a complete frame. Typically a new frame is addressed at least about 50 times per second to generate what appears to the human eye a flicker-free video image.
Typically the energy efficiency of such panels is fairly low as a result of the fact that each sub-pixel element has a relatively high electrical capacitance. When a range of voltages are simultaneously applied to the columns appropriate to address each row, the pixels on the remaining rows, which are electrically floating when they are not addressed, become partially charged. If there is a large number of rows, such as on a high resolution display, the ratio of energy expended in partially charging the non-addressed pixel as compared to the energy used to charge and activate the pixels on the addressed row can be quite large. Hence the overall energy efficiency of the display panel can be quite low, with a trend to lower efficiency as the resolution increases.
Minimizing the resistive loss associated with pixel charging can increase the energy efficiency of an electroluminescent display. This loss can be minimized by minimizing the peak charging current, and by minimizing the resistance of elements in the charging circuitry Generally, the former condition is realized when the pixels are charged at constant current. The energy efficiency can also be improved by a partial recovery of the stored capacitive energy in the pixels, but this is complicated by the fact the effective panel capacitance is strongly dependent on the extent of partial charging of the pixels on the non-addressed rows.
A variety of approaches have been used for improving the efficiency of electroluminescent displays. U.S. Pat. No. 4,847,609 teaches a technique for minimizing the power consumption of an electroluminescent display by a judicious choice of the thickness of the phosphor films and the capacitance of the dielectric layers used for the display. U.S. Pat. No. 5,856,813 teaches a system for reducing power consumption by maintaining the column voltage on certain rows in the event that the same column voltage is required on that row during successive frames. This scheme requires a complex feedback system that compares the image data for successive frames. U.S. Pat. No. 5,517,207 discloses the use of a three component driving voltage for an electroluminescent display whereby one of the voltage components is applied to all pixels to reduce the power dissipation in non-illuminated pixels. A more efficient display driver is set forth in U.S. patent application Ser. No. 09/504,472 wherein energy recovery is optimized and resistive losses are minimized. Although the above methods result in measurable improvement in operational efficiency of electroluminescent displays, further improvement is required before such displays are able to provide a competitive alternative to traditional CRT video display technology. The inventors have recognized that one area for deriving such an improvement is to reduce the relative energy loss associated with the non-addressed pixels.